1. Field of the Invention
The present invention relates to a negative pressure type booster device for vehicles and particularly, to a negative pressure type booster device capable of being highly responsive when a brake pedal is stepped on strongly.
2. Discussion of the Related Art
Generally, in negative pressure type booster devices, when the stepping of a brake pedal advances an input rod which thus causes a plunger to advance relative to a valve piston, a negative pressure valve comes into contact with a negative pressure valve seat to block the communication between a variable pressure chamber and a constant pressure chamber. When the plunger is further advanced then, an atmosphere valve seat is separated from an atmosphere valve, whereby the atmospheric air is led from the surrounding atmosphere to the variable pressure chamber through a silencer and a filter element. Thus, the valve piston is moved forward in dependence on a pressure difference between the variable pressure chamber and the constant pressure chamber to push a master piston, whereby a brake pressure corresponding to the steeping force of the brake pedal is generated in the master cylinder.
Since the valve piston elastically deforms a reaction member by an operation power corresponding to the pressure difference between the variable pressure chamber and the constant pressure chamber to push the master piston, the reaction member under the elastic deformation pushes the plunger rearward. This causes the plunger to retract, so that the atmosphere valve seat is seated on the atmosphere valve to block the communication of the variable pressure chamber with the atmosphere and hence, to hold the brake oil pressure at a desired pressure.
Although it is required to lead the atmospheric air to the variable pressure chamber without delay when the brake pedal is stepped on strongly, the use of the silencer makes it difficult due to an air path resistance thereacross to lead a sufficient volume of the atmospheric air to the variable pressure chamber, and this makes a cause of difficulty in enhancing the responsiveness in braking operation. Particularly, improvement has been required these days with an increasing desire for shortening the time for vehicle to stop after braking.
As one satisfying such desire, there has been known a negative pressure type booster device as described in U.S. Pat. No. 5,056,413 (equivalent of Japanese Examined Published Patent No. 6-24922). In the device described in the patent, a sleeve is provided around a valve housing (valve piston), and an additional annular air passage is defined between the sleeve and the valve housing to open to a rearward chamber (variable pressure chamber). An additional valve member is formed at the rear end of the sleeve and is biased toward a third valve seat so that the same is usually held closed by the additional valve member. When a brake pedal is stepped on sharply to advance an operating member (input rod) beyond a predetermined distance relative to the valve housing, the additional valve member formed at the rear end of the sleeve is separated from the third valve seat. This causes the atmospheric air from a filter element not only to flow into the rearward chamber through an inner space and an intermediate space, but also to flow into the rearward chamber through the additional air passage. As a consequence, the pressure in the rearward chamber rapidly comes to be equal to the atmospheric air, so that the braking responsiveness can be enhanced.
However, in the device described in the aforementioned patent, an additional sleeve defining the additional air passage is provided around the valve housing, and the sleeve is advanced when the brake pedal is stepped a large stroke, whereby the valve member is opened over the entire circumference thereof to admit the atmospheric air from the additional passage. However, since no silencer is provided in the additional air passage for quick admission of the atmospheric air, it would be probable that a throttling noise is made to leak outside when a massive volume of air passes through a throttle portion of an inner valve mechanism at a time upon the rapid opening of the valve member.
In addition, in the device described in the aforementioned patent, since a slight increase in the relative displacement between the sleeve and the valve member after the opening of the valve member brings about a sharp increase in output power, the responsiveness represents the operational modes of two kinds including an ordinary operation state and a responsiveness enhancing valve operation state, thereby giving rise to a problem that there cannot be obtained a responsiveness enhancing property which varies in dependence on the stepping state of the brake pedal.